The disclosure relates generally to turbine blade machining, and more particularly, to a method and fixture for hole drilling an elastically deformed superalloy turbine blade.
Large turbine blades for power generation turbines can include turbine blades more than one meter in length that are very thin relative to their span and chord, particularly near the trailing edge in radially outer span locations. Such turbine blades can also have curvature in the span-wise and chord-wise directions. Due to the high operating temperatures, turbine blades typically include a number of cooling passages extending therethrough, which are provided, among other reasons, to extend the creep life of the blades. Creep is effectively the long term accumulation of plastic strain that eventually leads to rupture. However, the turbine blade thinness and curvature can make it difficult to fabricate cooling passages or holes within the airfoil of the blade, including span-wise cooling passages that follow the curvature of the airfoil. Methods to fabricate long cooling passages with variable curvature, particularly accurately-placed long variable-curvature passages formed using widely employed shaped tube electrolytic machining (STEM) drilling, have not been developed.
One approach to produce curvilinear holes is to flatten the airfoil of the blade in the region where the hole is required, drill a straight hole and then bend the drilled region into the required curve. This process thus requires plastic deformation and reformation of the airfoil. Advances in turbomachinery technology however have led to the use of more advanced materials such as superalloys like high gamma prime superalloys, which cannot be deformed in this manner. In particular, plastically deforming superalloys, i.e., bending the material such that it does not automatically return to its original state, induces plastic strain and dislocations in the material that are not repaired when the superalloy is bent back into its original position. Conventional materials can be exposed to a high temperature thermal process in order to negate the impact of the strain and dislocations on the creep life. However, with superalloys exposed to a high plastic strain, the high temperature thermal process may generate recrystallized grains, weakening the material. Any manufacturing process that induces plastic strain into a turbine blade will effectively give the creep a head start and reduce the overall life of the blade. Consequently, this approach is inapplicable to current turbine blades that are made of superalloys.